1. Field of the Invention
The present invention relates to printed circuit boards, and particularly to a routing scenario of transmission lines of a printed circuit board capable of reducing crosstalk effect between the transmission lines.
2. Description of Related Art
Advancements in the electronics industry have brought ever smaller printed circuit boards (PCB) with increasing numbers of components. As a result, a PCB with a limited area requires disposing denser transmission lines thereon. This narrows the spacing between transmission lines and increases the crosstalk between transmission lines, possibly causing electronic components coupled to the transmission lines to take false actions.
Referring to FIG. 3, a routing scenario of three transmission lines of a conventional PCB is illustrated. As shown, the PCB includes signal layers f1, f2, transmission lines a1, a2, a3, vias b1, b2, b3, c1, c2, c3, and two electronic components 100, 200 laid on the signal layer f1. The transmission lines a1, a2, a3 are firstly routed from the electronic component 100 and in parallel to each other on the signal layer f1, then subsequently their paths are shifted to the layer f2 after respectively passing through the vias b1, b2, b3, and then return to the layer f1 after respectively passing through the vias c1, c2, c3. Finally, the transmission lines a1, a2, a3 are coupled to the electronic component 200.
According to known electromagnetic theory, crosstalk received by a transmission line is directly proportional to the quantity and lengths of adjacent transmission lines, and inversely proportional to the spacing between the transmission line and adjacent transmission lines. In FIG. 3, for simplifying computing, suppose lengths of the transmission lines a1, a2, a3 routed on the signal layer f1 are L1, lengths of the transmission lines a1, a2, a3 routed on the signal layer f2 are L2, and the spacing between adjacent transmission lines a1, a2, and a2, a3 is S, and L2=2*L1, the crosstalk XTKa1, XTKa2, XTKa3 received respectively by the transmission lines a1, a2, a3 are found using the following relational expressions:XTKa1=XTKa3XTKa3∝2L1/S2+2L1/(4S2)+L2/S2+L2/(4S2)=2.5*L2/S2 XTKa2∝2L1/S2+2L2/S2+2L1/S2=4*L2/S2 As shown above, as spacing S increases, crosstalk reduces. However, with ever-smaller PCBs, increasing spacing between transmission lines is not practical.
What is needed, therefore, is a PCB with reduced crosstalk effect between transmission lines therein.